TEMPERATURE COMPENSATION IN GREYSCALE ADDRESSING

Abstract: A method of addressing a liquid crystal pixel which has a varying switching threshold (1, 3) over its area comprises blanking the pixel by means of a blanking pulse (6), switching on part (10) of the area of the pixel by means of a second pulse (8) and then switching off a part (14) of the area (10) which was switched on by the second pulse (8) by means of a third pulse (12). The amount (14) of the pixel switched off by the third pulse has the same temperature dependence as the amount (10) switched on by the second, thus compensating for any change in brightness or grey level which would otherwise occur.

Description: TEMPERATURE COMPENSATION IN GREYSCALE ADDRESSING This invention relates to a method of addressing an optical cell which comprises a layer of material sandwiched between a pair of electrodes, the material having an optical property which is switchable from a first stable state to a second stable state by applying a voltage of one polarity and a given duration between the electrodes and from the second stable state to the first stable state by applying a voltage of the other polarity and the given duration between the electrodes, the magnitude of the voltage required between the electrodes to switch the optical property from either stable state to the other stable state being subject to different thresholds for different parts of the total area of the layer, which thresholds vary with temperature, in which method a first voltage is applied between the electrodes, the first voltage having the one polarity and a magnitude and duration which are appropriate to ensure that the optical property attains the second stable state over the total area of the layer, after which a second voltage is applied between the electrodes, the second voltage having the other polarity and a magnitude and duration which are appropriate to ensure that, at a given temperature, the optical property is switched by the second voltage from the second stable state to the first stable state over only a portion of the total area of the layer. A method of the above general kind is disclosed, for example, in EP-A-0240010. Cells containing material which is electrically addressable to change its optical property, for example between a light-transmissive state and a non light-transmissive state, are commonly proposed for use in displays or printer applications. An array of such cells may be formed, for example, by means of a pair of transparent substrates sandwiching a layer of ferroelectric liquid crystal material between them, and each carrying a set of transparent electrodes oriented so as to cross each other to define a matrix of pixels. In such a case each pixel can be addressed by applying an electrical signal to the corresponding member of each set of electrodes. Pixels with varying switching thresholds over their areas may be formed, for example, with one or both sets of electrodes having varying thicknesses across their width so that an applied voltage between overlapping electrodes produces differing electric fields across the width of the pixel, which may be sufficient to cause switching of the material in some areas but not in others. As an alternative they may be formed, for example, using an alignment control layer which has different alignment control powers at different regions over the area of each pixel. Both of these possibilities are disclosed in the aforementioned EP-A-0240010. The variation is preferably continuous; if the variation has a stepped form the steps are preferably small and many in number. A problem with prior art methods of addressing such matrices is that the switching threshold of the material may vary with temperature, so that for a given applied voltage, as the temperature varies so does the amount of the pixel which switches, and thus the brightness level or grey level obtained. It is an object of the present invention to alleviate this problem of the known prior art. According to the present invention, a method as defined in the first paragraph is characterised in that, after the application of the second voltage, a third voltage is applied between the electrodes, the third voltage having the one polarity and a magnitude and duration which are appropriate to ensure that, at the given temperature, the optical property is switched back by the third voltage to the second stable state over only a portion of that portion of the total area of the layer over which the optical property has been switched to the first stable state by the second voltage. The switchings by the second and third voltages may have substantially the same temperature dependance, so that the amount of variation of the brightness level with temperature can be reduced. If the second voltage has a magnitude and duration which, at a first further temperature different from die given temperature, are appropriate to just ensure that the optical property is switched by the second voltage from the second stable state to the first stable state over the total area of the layer, an increase in the temperature range over which a reduction in brightness level variation with temperature can be obtained may be achieved by arranging that, after the application of the third voltage, a fourth voltage is applied between the electrodes, the fourth voltage having the other polarity and a magnitude and duration which are appropriate to just fail to ensure that, at the first further temperature, the optical property is switched by the fourth voltage from the second stable state to the first stable state over any portion of the total area of the layer. In such a case, if the third voltage has a magnitude and duration which, at a second further temperature different from the given temperature and the first further temperature, are appropriate to just ensure that the optical property is switched back by the third voltage to the second stable state over the total area of the layer, a further increase in the temperature range may be achieved by arranging that, after the application of the fourth voltage, a fifth voltage is applied between the electrodes, the fifth voltage having the one polarity and a magnitude and duration which are appropriate to just fail to ensure that, at the second further temperature, the optical property is switched by die fifth voltage from the first stable state to the second stable state over any portion of the total area of the layer. The material may be ferroelectric liquid crystal material. In order that the present invetnion may be more readily understood, reference will now be made, by way of example, to me accompanying diagrammatic drawings, in which: Figure 1 shows the pulses applied during one addressing of a cell in to one embodiment of the invention; Figure 2 is graph showing light transmission against time for the cell throughout the addressing period for two different temperatures; Figure 3a is a plan view of the cell after the second voltage has been applied, for two different temperatures; Figure 3b shows the cell after the third voltage has been applied; Figure 4 is a graph showing the electric field experienced across the material of a cell for four different voltage pulses in another embodiment of the invention; Figure 5 shows plan views of the cell after each pulse in Figure 4 has been applied, for four different temperatures; Figure 6 shows a matrix of cells together with addressing means therefor; Figure 7 shows signals which may be produced by the addressing means of Figure 6; Figure 8 also shows signals which may be produced by the addressing means of Figure 6; and Figure 9 shows signals which may replace the signals of Figure 6. Referring to Figures 1 and 2, an optical cell (not shown) comprising a layer of ferroelectric liquid crystal material sandwiched between a pair of electrodes, which cell is constructed in such manner that the magnitude of the voltage N required to switch the material from one of its stable optical states to the other, when this voltage has a given duration, is subject to different thresholds for different parts of the total area of the layer, is addressed by applying three voltage pulses 6,8 and 12 respectively between the electrodes in succession. The pulses 6 and 12 have one polarity and the pulse 8 has the other polarity. This example shows the inverse mode of operation, where switching from one stable state to the other occurs below a threshold voltage level, and does not occur above that threshold level. The switching threshold, given a voltage pulse of a certain widdi, varies across the cell from a first level 1,1' at one edge of d e pixel (2 in Figures 3a and b) to a greater second level 3,3' at die other end (4 in Figures 3a and b). The first pulse 6 sets the whole of the ferroelectric material of the cell to its second stable optical state (which may correspond to a dark or non light-transmissive state of a pixel of which the cell may form a component if it is situated, for example, between crossed polarisers). The second pulse 8, the voltage level of which lies between the respective switching thresholds 1, 3 at the edges 2, 4 of the cell, then sets to its first stable optical state a part 10 of the liquid crystal layer adjacent the other end 4, whilst leaving a part 11 at the one end 2 unswitched. (The first stable state may correspond to die light or light-transmissive state of the aforementioned pixel). When the third pulse 12 of the opposite polarity to the second is applied, die voltage level of diis pulse also lying between the respective switching thresholds 1', 3' at the edges 2, 4 of die pixel, a portion 14 of the switched part 10 of the layer is switched back to d e second state whilst the part adjacent d e edge 2 is unaffected. In the example it is required to switch half of a pixel of which the cell forms part to the light-transmissive state. The second voltage pulse 8 exceeds die switching threshold of about a quarter of the widdi of the pixel, so that this part remains in the dark state whilst three quarters of die pixel switches to the light-transmissive state. The third pulse 12 is applied shortly thereafter, and exceeds the switching threshold of three- quarters of the widdi of die pixel which is thus unaffected, whilst one quarter switches back to the dark state. Therefore the brightness level 15 achieved is half of the total possible brightness level. Should the temperature rise, the switching threshold of die material over the entire widdi of the pixel becomes greater; iat is, more positive or more negative, as shown in broken lines in Figure 1. Thus the second voltage pulse 8 causes a greater part 10 of the pixel to switch to the light state whilst the diird voltage pulse 12 causes a greater portion 14 of that part to switch back to the dark state, as shown in broken lines in Figures 3a and b. It can be seen that, whilst the brightness level 13 after the second voltage pulse changes with temperature as shown in broken lines in Figure 2, the third voltage pulse compensates for this change, and d e resulting level 15 may be the same, as shown in Figures 2 and 3b. It will be evident diat other brightness levels may be obtained by appropriately choosing the magnitude of d e pulse 8 and/or 12. Consideration of Fig 1 reveals diat, should die temperature rise still further, the threshold 1 may eventually coincide witii or even lie above die top of die pulse 8, with die result diat die pulse 8 will switch the whole of the pixel to the light-transmissive state. Compensation for any change in threshold will not occur when this situation is present unless further steps are taken. A possible such further step will now be described witii reference to Fig 4. As shown in Fig 4, the third voltage pulse 12 may be followed by a fourth voltage pulse 34, of the same polarity as the second pulse 8, applied between the electrodes of the cell. Moreover, if desired, die fourth pulse 34 may be followed by a fifth voltage pulse 36 of the same polarity as the ti ird pulse 12, also applied between die electrodes of die cell. In Fig 4 the sloping top of each voltage pulse 8, 12, 34, 36 is not intended to indicate diat the magnitude of the pulse decreases with time (as this is not the case) but rather represents the effective electric field experienced by the ferroelectric liquid crystal material from one edge of the cell to the other during d e application of die relevant voltage pulse thereto. This example shows the 'normal mode' of operation of switching the material; that is operation in a voltage range in which a low voltage does not cause switching whilst a higher voltage does cause switching. The lines T, to T 4 represent the switching threshold of die material at increasing temperatures for the pulse widths shown, and relate also to Fig 5. The lines T A and T B represent the switching threshold of die material at particular temperatures within this range and will be referred to below. After blanking with a first voltage pulse (not shown) as before and die application of second and third voltage pulses 8 and 12 as before, fourth and fifth voltage pulses 34 and 36 are applied across d e electrodes of die cell. In this example the second and fourth voltage pulses 8 and 34 are constituted by fixed-magnitude voltage pulses V^ and Vs 34 respectively applied to one electrode of die cell combined with variable-magnitude but mutually equal data voltage pulses N D8 and N D34 respectively applied to die otiier electrode of the cell. The third and fifth voltage pulses 12 and 36 have fixed magnitudes. It will be seen that the lowest electric field experienced by any part of the material of the cell due to d e application of die pulse 8 coincides witii the threshold line T A , as does die highest electric field experienced by any part of the material of the cell due to the application of die pulse 34. Similarly the lowest electric field experienced by any part of the material of the cell due to the applicaiton of the pulse 12 coincides with die threshold line T B , as does the highest electric field experienced by any part of the material of the cell due to die application of die pulse 36. Thus, at the temperature at which the threshold T A is applicable, the pulse 8 has a magnitude and duration which just ensure that the material of die cell is switched by the pulse over the whole of its area to an optical state corresponding to a a bright or light - transmissive state of a pixel of which the cell forms part, and the pulse 34 has a magnitude and duration which just fail to ensure that the material of the cell is switched by the pulse over any portion of its area to this optical state. Similarly, at the temperature at which the threshold T B is applicable, the pulse 12 has a magnitude and duration which just ensure that the material of the cell is switched by die pulse over the whole of its area to an optical state corresponding to a dark or light non-transmissive state of the pixel, and d e pulse 36 has a nagnitude and duration which just fail to ensure that the material of the cell is switched by die pulse over any portion of its area to this optical state. Referring also to Fig 5 it can be seen that when, for example, a brightness level of one half of the possible brightness is required, at temperatures at which the thresholds T, and T 2 apply the second and tiiird pulses 8, 12 have an effect similar to mat already described, and die fourth and fifth pulses 34, 36 do not have any effect since the electric field experienced by any part of the ferroelectric material due to these pulses lies entirely below the switching threshold. At a higher temperature at which the threshold T 3 , applies, the second pulse 8 hes entirely above the switching threshold and tiierefore switches the entire pixel. At this temperature, part of die fourth pulse 34 lies above the threshold so that a part x 5 of the pixel is switched by this pulse. In the same way, at a still higher temperature T 4 , the third pulse 12 switches the entire pixel by lying entirely above the threshold. At tiiis temperature the fifth pulse 36 come into effect to switch a part x-s of the pixel. In this manner, the fourth and fifth pulses 'take over' from the second and tiiird pulses when die temperature is such that the latter cease to have a brightness controlling effect. It will be appreciated d at still further pulses of alternating polarity similar to the pulses 34 and 36 and each having the same properties relative to the immediately preceding pulse of die same polarity as do die pulses 34 and 36 to the pulses 8 and 12 respectively could be applied to the cell to enable a still broader temperature range to be accommodated, if desired. Although as described d e brightness ultimately achieved is controlled by means of variable componenets V Dg and V D34 superimposed on the fixed components V j g and Vs 34 of the pulses 8 and 34 respectively, brightness control may alternatively be achieved by means of variable components superimposed on die fixed components of the pulses 12 and 36. A further possibility is to superimpose variable components on the fixed components of the pulses 12 and 36 in addition to die variable components superimposed on the fixed components of the pulses 8 and 34, die variable componenets of the pulses 12 and 36 then bearing a predetermined relationship to the variable components of the pulses 8 and 34. As mentioned previously, the example of Figs 4 and 5 relates to the "normal mode" of operation of switching the material of the cell, i.e. operation in a voltage range in which a low voltage does not cause switching whereas a higher voltage does cause switching. For operation in the "inverse mode", in which the opposite is the case, die successive pulses of die same polarity, e.g. pulses 8 and 34 and pulses 12 and 36, should have increasing rather than decreasing magnitudes. Moreever the magnitude of the pulse 12 should then be greater than diat of die pulse 8, and die magnitude of the pulse 36 should tiien be greater than the magnitude of d e pulse 34. Although as described with reference to Figs 4 and 5 control of the brightness ultimately obtained is achieved by controlling the magnitudes of die pulses 8 and 34, and/or the magnitudes of the pulses 12 and 36, it will be appreciated d at such control may alternatively or additionally be achieved by controlling the widdis of these pulses. The invention has been described so far in the context of addressing a single optical cell. As previously mentioned, an array of such cells may be formed, for example, by means of a pair of transparent substrates sandwiching a layer of ferroelectric Uquid crystal material between them and each carrying a set of transparent electrodes oriented so as to cross each other to define a matrix of pixels. Some possible ways in whcih the cells of such a matrix may be addressed by a method in accordance witii the invention will now be described. Figure 6 of the drawings shows such a matrix together with addressing means therefor in diagrammatic form. More particularly it shows a matrix-type array 41 of Uquid crystal cells comprising a pair of transparent plates which are superimposed one upon die od er with a small spacing therebetween which contains ferroelectric Uquid crystal material. The array comprises a plurality of picture elements (pixels) in the form of ceUs which are defined by areas 42 of overlap between members of a first set of parallel transparent electrodes 44 provided on die inner surface of one plate, i.e. on one side of the Uquid crystal material, and members of a second set of parallel transparent electrodes 43 provided on die inner surface of the other plate, i.e. on the other side of die Uquid crystal material. The electrodes 43 and d e electrodes 44 cross each other and in the present example are oriented substantially orthogonal to each other and each corresponds to a respective Une of pixels. (With the orientation shown each electrode 43 corresponds to a respective column of pixels and each electrode 44 corresponds to a respective row). As previously discussed with reference to a single cell, die array is constructed in such manner that the voltage required between the electrodes corresponding to each pixel to switch die ferroelectric material of that pixel from one stable state to the other is subject to different tiiresholds for different parts of the area of the pixel, which thresholds vary with temperature. The array 41 is addressed by means of an addressing signal generator 45 via conductors 46 which are connected to respective electrodes 43 and conductors 47 which are connected to respective electrodes 44. For each pixel the resulting electric field appUed thereacross determines the aUgnment of the Uquid crystal molecules over the various parts of the pixel and hence the optical states of the various parts of that pixel. The array 41 is positioned between parallel or crossed polarizers (not shown). The orientation of d e polarizers relative to die aUgnment of the Uquid crystal molecules determines whether or not Ught can pass through a pixel when the ferroelectric material thereof is in a given state. Accordingly, for a given orientation of the polarizers, the various parts of each pixel each have a first and a second opticaUy distinguishable state provided by die two stable states of the Uquid crystal molecules included in the relevant part of that pixel. The signals produced by generator 45 may be as shown in Figs 7 and 8. Fig 7 shows at 70 die complete waveform appUed to each of the conductors 47 of Fig 6 (although staggered in time from conductor to conductor) by the generator 45 in order to address the row of pixels 42 corresponding to that conductor. The vertical dashed Unes signify successive time periods of equal length ("slots"). Fig 7 moreover shows at 71 data waveforms which are appUed in parallel to aU the conductors 46 with the time relationships to the waveform 70 indicated. Each complete waveform 70 comprises, similarly to what has already been described with reference to Figs 1 and 4, first, second, tiiird, fourth and fifth fixed-amptitude voltage pulses 6, 8, 12, 34 and 36 respectively. The pulse 6 sets each and every pixel on the corresponding row to the blanked state, whatever waveform 71 is simultaneously appUed to die conductors 46. Each data waveform comprises a pulse 72 of one polarity, a given magnitude and a duration equal to one time slot, a pulse 73 of the other polarity, the given magnitude and a duration equal to one time slot, and a portion 74 of zero voltage and a duration equal to two time slots. The first and third of die data waveforms shown carry data for d e pixels of the row to which the waveform 70 is appUed whereas the second and fourth of the data waveforms shown carry data for pixels of another row or rows, as wiU become evident hereinafter. Although each of the data waveforms is shown as being identical this in fact will not normally be the case. Nor will it normaUy be the case diat d e data waveforms being appUed in paraUel at any given time to all the conductors 46 wiU be identical as each has to carry data for a respective pixel on the currently addressed row, and this data wiU not normally be the same for each pixel. Although not shown in the drawing d e data carried by a given data waveform on a given conductor 46 is made variable by making the magnitude of its two voltage pulses 72 and 73 variable and also tiieir polarities (altiiough they will always be of equal magnitude and mutually opposite polarities in order to maintain so-called "charge balance"). It will be seen that the (fixed- ampUtude) second and fourth pulses 8 and 34 of waveform 70 each coincide witii die (variable magnitude and polarity) first pulse 72 of a respective data waveform so that the resulting voltages across the pixel at the intersection of the corresponding electrodes 43 and 44 are a combination of tiiese pulses - c.f. the fixed component V s and die variable component V D of the pulses 8 and 34 in Fig 4 -, whereas the (fixed-amplitude) third and fifth pulses 12 and 36 of waveform 70 each coincide witii the zero voltage portion 74 of a data waveform, so that d e resulting voltages across the pixel at the intersection of die corresponding electrodes 43 and 44 are simply due to tiiese fixed-aptitude pulses -c.f. die fixed-amplitude pulses 12 and 36 of Fig 4. Fig 8 illustrates how the waveforms 70 appUed by generator 45 to respective areas of the row conductors 47 of Fig 6 may be related in time, both to each otiier and to successive data signals N d appUed by generator 45 in paraUel to the conductors 46. The blanking pulses 6 have not been shown in Fig 8 for clarity's sake. The waveforms 70 appUed to four successively addressed rows of pixels via respective ones of die conductors 47 are denoted by n - 1, n, n + 1 and n + 2 respectively. Fig 9 shows a possible alternative to the waveforms 70 and 71 of Fig 7. In Fig 9 the waveforms 70 and 71 of Fig 7 are replaced by die waveforms 90 and 91 respectively. The differences between the waveforms 70 and 90 is diat, in waveform 90, a further voltage pulse 12A, equal in magnitude and duration to the voltage pulse 12, precedes the voltage pulse 12 at such a time diat it is spaced from the pulse 12 by one time-slot, and a futher voltage pulse 36A, equal in magnitude and duration to the voltage pulse 36, precedes d e voltage pulse 36 at such a time that it is spaced from die pulse 36 by one time-slot. The data waveforms 91 each include two pulses 92 and 93 of opposite polarity similar to the pulses 72 and 73 of die waveforms 71 of Fig 7. However the pulses 93 are spaced from the immediately preceding pulses 92 by one time-slot, rather than foUowing them directly. Thus each of the pulses 12, 12A, 36 and 36A coincides with a zero, 94 or 95, in a data waveform 91. If desired die time relationship between the waveforms 90 and 91 of Fig 9 may be changed by shifting each of the pulses 12, 12A, 36 and 36A one time-slot forward in time. If this is done each of tiiese pulses will coincide with a non-zero part of a data waveform 91. However one of the pulses of each pair 12, 12A and 36, 36 A wiU coincide witii a pulse 92 of a given data waveform whereas the otiier will coincide witii the pulse 93 of the same waveform. As these two pulses 92 and 93 have the same magnitude but opposite polarities the average effect of these pulses on the pulse pair 12, 12A or 36, 36A wiU be negUgible.

Claims: CLAIMS 1. A method of electricaUy addressing an optical ceU (42) which comprises a layer of material sandwiched between a pair of electrodes (43, 44), die material having an optical property which is switchable from a first stable state to a second stable state by applying a voltage of one polarity and a given duration between the electrodes (43, 44) and from die second stable state to d e first stable state by applying a voltage of the other polarity and die given duration between the electrodes (43, 44), die magnitude of the voltage required between the electrodes to switch die optical property from either stable state to the other stable state being subject to different thresholds (1,3) for different parts of the total area of the layer, which thresholds (1,3) vary with temperature, in which method a first voltage (6) is applied between the electrodes (43, 44), die first voltage (6) having the one polarity and a magnitude and duration which are appropriate to ensure that the optical property attains the second stable state over the total area of the layer, after which a second voltage (8) is appUed between the electrodes, die second voltage (8) having the other polarity and a magnitude and duration which are appropriate to ensure that, at a given temperature, the optical property is switched by die second voltage (8) from the second stable state to the first stable state over only a portion (10) of the total area of the layer, characterized in that, after the appUcation of the second voltage (8), a tiiird voltage (12) is applied between die electrodes (43, 44), die tiiird voltage (12) having the one polarity and a magnitude and duration which are appropriate to ensure that, at the given temperature, the optical property is switched back by the third voltage (12) to the second stable state over only a portion (14) of that portion (10) of the totalarea of the layer over which die optical property has been switched to die first stable state by the second voltage (8). 2. A metiiod as claimed in Claim 1 wherein the second voltage (8) has a magnitude and duration which, at a first further temperature different from the given temperature, are appropriate to just ensure that the optical property is switched by the second voltage (8) from the second stable state to the first stable state over the total area of the layer, in which method, after the appUcation of the third voltage (12), a fourth voltage (34) is applied between the electrodes (43, 44), die fourth voltage (34) having the other polarity and a magnitude and duration which are appropriate to just fail to ensure that, at the first further temperature, the optical property is switched by die fourth voltage (34) from the second stable state to the first stable state over any poriton of the total area of the layer. 3. A method as claimed in Claim 2 wherein the third votlage (34) has a magnitude and duration which, at a second further temperature different from the given temperature and d e first further temperature, are appropriate to just ensure that die optical property is switched back by the third voltage (34) to the second stable state over the total area of the layer, in which method, after the appUcation of d e fourth voltage (34), a fifth voltage (36) is appUed between the electrodes (43, 44), the fifth voltage (36) having the one polarity and a magnitude and duration which are appropriate to just fail to ensure that, at the second further temperature, the optical property is switched by die fifth voltage (36) from the first stable state to the second stable state over any portion of the total area of the layer. 4. A method as claimed in Claim 3, wherein the material is ferroelectric Uquid crystal material.

Inventors: LOCKMULLER NEIL, LOCKMULLER, NEIL
Applicants: CENTRAL RESEARCH LAB LTD, LOCKMULLER NEIL, CENTRAL RESEARCH LABORATORIES LIMITED, LOCKMULLER, NEIL
Application year: 1994
Main classification: G09G3
IPC codes: G02F1, G09G3